In theme with the Mets attempting to make the NL Wild Card spot during the 2019 MLB season, despite grossly underperforming before the All Star break, what has been the biggest turnaround for a team not in contention for post season play to make the playoffs?

Please include any relevant team information and if there was a specific reason for this change of fortune.

Not that I'm knowledgeable enough to provide an answer, but what is a turnaround? In '07 the Rockies were thought to be out of the race after they lost 3 in a row with over two weeks left in the season. Then they won 13 of their last 14 regular season games to get into a playoff for the wild card, won that, won the wild card, and went on to the World Series. Does this constitute a turnaround (even though they were still 5 games over .500 when they started that run)? Or do you want something to start at the All Star break (if so, what about before there was an All Star game)? Some other point?
– GreenMattAug 7 '19 at 14:12

I assume you mean what's the most games behind an MLB team ever was for a player spot that they wound up winning.
– DJ Spicy DeluxeAug 12 '19 at 23:14

The Miracle Braves were 15 games out of first place on July 6 but went on to go 68-19 in the final half of the season and won the pennant by 10 games.

The Wikipedia page goes into more details.

The Braves performed one of the most memorable reversals in major league history, going from last place to first place in two months, becoming the first team to win a pennant after being in last place on the Fourth of July. After finishing in fifth place in 1913 with a record of 69 wins and 82 losses, the Braves were not expected to be contenders. They spent the first part of the season in last place, posting a record of 26 wins and 40 losses in early July. Led by three pitchers, Dick Rudolph, Bill James, and Lefty Tyler, the team began to win games, taking over first place for good on September 8. Their record over their final 87 games was 68–19 for a winning percentage of .782. Infielders Johnny Evers and Rabbit Maranville led the league in double plays. The Braves went on to sweep Connie Mack's heavily favored Athletics in four games in the 1914 World Series. The team became known as the "Miracle" Braves and remain one of the most storied comeback teams in baseball history.

The 2012 Oakland Athletics are on this list! They had not been in first place at all the entire year. They had an unbelievable run from June to September but still themselves 5 games out of a postseason berth with 9 games left to play. After defeating the Rangers in Gm 161 - they found themselves tied for first in the AL West. The next day they won gane 162, not only earning a playoff berth but winning the division. It was the only day in the 2012 season that they were in first place - and the thing is, that is the only day that it even matters.

They went on to lose the division series to the Tigers and mainly Verlander - the series went to a full 5 games.